An aerosol is understood to mean particles in liquid or solid phase suspended in air or a carrier gas in the airborne state. The aerosol is considered to be a disperse system formed of solid or liquid particles that are finely distributed in air or a carrier gas.
Aerosols are characterized by single basic features. A single individual aerosol particle is described by three features, specifically shape, size and substance. The aerosol as an accumulation of many individual particles or as a particle collective is described in greater detail by further properties, specifically concentration and particle size distribution.
Optical particle sensors often work with electromagnetic radiation in a wavelength range of from 600 nm to 780 nm.
The wavelength range of 380 nm to 780 nm is also referred to as light, since it lies within the range perceived by the human eye.
Hereinafter, the term “light” will therefore also be used instead of the term “electromagnetic radiation”, since the term “light” includes the range of electromagnetic wavelengths usual for optical particle sensors.
A wavelength of approximately 655 nm is often used, since there are very economical laser diodes with this wavelength as a source for the required light.
In order to measure the particle mass concentration, aerosol photometers (APMs) are used that are also referred to in the technical literature as “light-scattering nephelometers”.
Aerosol photometers measure the concentration in a particle collective. The measurement result is the particle mass concentration. This is often specified in mg/m3.
Due to their operating principle, aerosol photometers can be used with particle mass concentrations up to several 100 mg/m3.
Either laser diodes or light-emitting diodes (LEDs) are used as monochromatic light sources for aerosol photometers. LEDs are used in economical aerosol photometers. In principle, optical smoke detectors for example fall under the group of aerosol photometers.
In the case of aerosol photometers a zero-point adjustment must be made regularly, since contamination and ambient influences lead to a drift of the zero point. High-quality aerosol photometers are provided with means so as to be able to perform this zero-point adjustment automatically. To this end, the aerosol is firstly guided through a filter or over a separator, so that there are no longer any detectable particles in the measurement volume. The “correction value” then recorded is stored and subtracted from the photometer measured values in the subsequent aerosol measurements. The difference is then output as the photometer measured value.
When it comes to taking ambient measurements in cities, aerosol photometers are suitable measurement apparatuses. In heavily loaded cities, partial particle mass concentrations of more than 0.4 mg/m3 are sometimes measured.
Another class of optical particle sensors is constituted by optical particle counters (OPCs). These measurement apparatuses also use the effect of light scattering in aerosols. However, in contrast to aerosol photometers, it is not a particle collective that is measured, but instead individual particles. To this end, the optical and electrical requirements are much higher than in the case of aerosol photometers. In the case of an aerosol photometer, the light scattered by thousands of particles is detected. Since, in the case of an optical particle counter, only the light scattered by an individual particle is detected, a much higher sensitivity and/or light intensity is necessary.
The optical measurement volume, which in aerosol photometers can easily be several 100 to 1000 mm3, has to be made much smaller in the case of optical particle counters. If, for example, 1000 particles per cm3 are to be measured error-free with an optical particle counter, the optical measurement volume must be only approximately 0.5 mm3 in size. It is thus ensured that only one particle is ever located in the optical measurement volume, up to a particle number concentration of 1000 particles per cm3. There are approximately 1000 particles per cm3 for example in Shanghai with a PM2.5 air load of 120 μg/m3.
At higher particle number concentrations, what are known as coincidence errors occur.
There are then a number of particles simultaneously in the optical measurement volume. These particles are then detected as an individual particle and are classified in an incorrect size class. This produces errors in the measurement result.
These coincidence errors mean that, in the above-mentioned case, this optical particle counter can no longer be used already from a relatively low load of 120 μg/m3.
Optical particle counters, however, do have some technical advantages with regard to their usable concentration range compared to aerosol photometers.
Optical particle counters do not have a zero-point drift, since a signal shape is assessed rather than a signal value.
Besides the number of particles, the particle size distribution (PSD) can also be detected on the basis of the signal shapes.
Optical particle counters calculate the particle mass concentration in an aerosol by dividing the detected particle sizes into size classes (bins) and measuring the frequency of occurrence for each size class or for each bin. Each size class or each bin is assigned a specific weighting factor that, multiplied by the frequency of occurrence, gives the particle mass for this size class or for this bin.
If the particle masses of all relevant size classes or bins are added together, this gives the total mass concentration.
In order to calculate PM2.5, the particle masses of all size classes or bins up to a particle size of 2.5 μm diameter are added together.
In order to calculate PM10, the particle masses of all size classes or bins up to a particle size of 10 μm are added together.
Optical particle counters respond in a much more robust manner to changes to the particle size distribution in the aerosol. If the particle size distribution in the aerosol changes towards large particles, the mass is underestimated with aerosol photometers, since the mass of a particle grows with the square of the surface area. The scattered light, however, is linear to the surface.
The aerosol photometers (APM) already mentioned above are measurement instruments of relatively simple structure. With a particle-measuring apparatus based on aerosol photometers of this kind, it is therefore generally only possible to estimate the particle mass concentration in the air or in the carrier gas, because the aerosol photometers always detect the optical properties of the totality of all aerosol constituents located in the measurement volume. Aerosol photometers measure a particle collective. No information regarding individual particles can be obtained from the detected signal of the scattered light.
In order to obtain the correct particle mass concentration from the detected signal, a key precondition must be met, specifically the complex properties of the aerosol to be measured as particle collective must correspond to those of the aerosol used for the calibration of the aerosol photometer.
The complex properties of an aerosol are particle size, particle size distribution, moisture, refractive index, and density.
It is first assumed that the complex properties of the aerosol to be measured are known and are constant.
The calibration is then performed for example as follows:
The aerosol photometer is calibrated with the aerosol to be measured. The procedures for this are described in detail in the handbooks of the aerosol photometers and in the instructions provided by the measurement apparatus manufacturer. Here, the mass concentration of the aerosol is always measured over a sufficient period of time with the factory calibration setting of the aerosol photometer, and the aerosol is guided after the aerosol photometer through a filter or over a separator. At the end of the measurement, the deposited mass is determined by weighing. The result of the weighing is not influenced by the optical particle properties, nor is it falsified by the particle size distribution or density differences of individual particles.
A correction factor K is then calculated from the two measured values obtained. This value K is stored in the aerosol photometer as a correction factor, also referred to as a calibration factor, or specific calibration value. The photometer measured values of the aerosol photometer are then multiplied by K in order to attain the correct particle mass concentration.
Problems occur when one or more of the complex aerosol properties of the aerosol to be measured change.
For example, the factory calibration factor can lead in environmental measurements to an overestimation of the particle mass concentration.
The factory calibration is performed using a test dust. The used test dust is known as “Arizona Road Dust A1”. This dust is standardized according to ISO12103A1.
The reason for the overestimation, inter alia, is the different density of the calibration dust compared to the average density of the dust particles in the ambient air.
The average dust density in the ambient air is not constant, and instead is dependent on many environmental factors. In the literature, a value of 1.6 to 1.7 g/cm3 is often found. The calibration dust, however, has a density of 2.65 g/cm3.